Hydrostatic pumps convert the mechanical energy transmitted by a prime mover into hydraulic energy through the pumping of hydraulic fluid. A common type of hydrostatic or hydraulic pump is an axial piston-type pump including a plurality of reciprocating pistons housed within a rotating pump barrel, and which are in fluid communication through hydraulic porting with system components or actuators. Rotation of the hydraulic pump barrel relative to a moveable swash plate creates an axial motion of the pump pistons that forces hydraulic fluid through the hydraulic porting to the other system components.
As referenced above, the pumping action of the pistons is realized by means of the pistons reciprocating axially in and out of a rotating cylinder pump barrel by interaction against a swash plate. The maximum rotational speed at which barrel chambers fill completely with working fluid under atmospheric pressure is called the “self-priming speed”. The self-priming speed is a significant parameter that provides a measure of performance of the pump. A higher self-priming speed means more output power, as power has a linear relationship to output flow (speed). Higher self-priming speed allows operation at higher speeds without cavitation, or operation at lower inlet pressure for a given speed such as is desirable at higher elevations.
Accordingly, it is desirable to achieve as high of a self-priming speed as possible to increase the output power. One option to increase the output power is simply to increase flow by increasing the speed of rotation of the pump barrel. However, increasing output flow merely by increasing the pump speed is limited by the filling capacity of the pump, which decreases with the pump speed due to decreased inlet pressure.
Other alternatives have been employed to increase output flow. For a given self-priming speed, a larger pump may be employed to increase the output power, but spatial considerations may preclude use of a larger pump. Multiple pump configurations also have been employed to generate a higher output power. In a typical configuration, a second impeller style pump may be employed in combination with a piston-barrel style hydrostatic pump. The use of the second impeller pump permits increasing the speed of rotation of the barrel by increasing fluid pressure at the inlet of the barrel, and otherwise can improve inlet conditions to prevent cavitation issues (which is particularly useful at high altitudes). The multiple pump configurations, however, have a disadvantage in that the number of components increases, which in turn increases the size, cost, and maintenance issues associated with the pump system. Accordingly, conventional mechanisms for increasing the output power of a hydrostatic or hydraulic pump have proven to be deficient.